An exhaust emission control apparatus that purifies and processes NOX to harmless components by injecting and supplying a liquid reducing agent or a precursor thereof to an exhaust upstream side of a NOX reduction catalyst arranged in an engine exhaust system to cause a catalytic reduction reaction between the NOX in the exhaust and the reducing agent has been proposed. In such an exhaust emission control apparatus, a technique for judging that a storage tank for storing the liquid reducing agent or the precursor thereof is empty, is described in Japanese Laid-open (Kokai) Patent Application Publication No. 2006-342782 (Patent Literature 1). In this technique, a concentration sensor that measures concentration of the liquid reducing agent or the precursor thereof based on a temperature rise characteristic of a heating element is used.
[Patent Literature 1] Japanese Laid-open (Kokai) Patent Application Publication No. 2006-342782